Wireless communication network protocols and standards continuously evolve to support ever-higher data rates. A known technique for increasing data rates is link adaptation, also known in the art as adaptive modulation and coding. In link adaptation, various signal and protocol parameters—such as the modulation technique, selection of forward error correcting (FEC) codes, and the like—are dynamically varied to match changing conditions of the radio link. Conditions of the radio link giving rise to link adaptation include path loss, interference from other signals, receiver sensitivity, available transmitter power margin, and the like. As an example of adaptive modulation technique, in the GSM system, packet switched channels may be modulated using GMSK, QPSK, 8 PSK, 16 QAM, or 32 QAM.
GPRS, EGPRS, and EGPRS2 systems transmit a radio block in the downlink comprising four bursts, where each burst is one 576 u-sec TDMA time slot. All bursts in the radio block are modulated using the same modulation technique. However, a link adaptation function in a base station transmitter may modulate different radio blocks within a temporary block flow (TBF) using different modulation techniques, depending on then-current channel conditions. The transmitter does not include an indication of the modulation technique in the downlink signal. Accordingly, it is unknown at the receiver.
The downlink signal does include a known training sequence in each burst. Blind modulation decision receivers demodulate the known training sequence using each allowed modulation technique. They then compare demodulation quality metrics to determine the most likely modulation technique, and independently demodulate each burst using that technique. After all bursts in a block are demodulated, the data are assembled and decoded.
While all of the bursts in a radio block are modulated using the same modulation technique, the receiver's modulation technique decisions for all bursts in a block do not necessarily agree. In conventional receivers, when a burst is demodulated using a modulation technique decision that does not match the modulation technique decision used for the other bursts in the block, the odd burst is erased by setting all the soft bits output by the demodulator to zero—effectively assigning each bit an equal probability of being a 0 or 1—and adjusting the number of bits in the burst according to the final decision of modulation technique for all the bursts in a radio block.
When one or more bursts in a block are erased due to an incorrect modulation technique decision, the probability of correctly decoding data decreases. In particular, the uplink state flag (USF) may be decoded incorrectly. The USF is the mechanism by which the network informs a mobile station which time slot to use for uplink transmission. If the mobile erroneously decodes the USF for another mobile as its own, it will transmit at the same time as another mobile station, causing interference at the base station, which may not be able to decode either uplink transmission. If the mobile fails to decode a USF intended for it, it will fail to transmit data when the base station is expecting it, wasting air interface resources and reducing the uplink data throughput. Modulation technique detection failure is the leading cause of USF decoding error.